1. Field of the Invention
The present invention relates to an ignition apparatus for use in an internal combustion engine, the apparatus including one or more spark plugs attached to corresponding cylinders of the internal combustion engine and being adapted to induce spark discharge at the spark plugs for igniting an air-fuel mixture.
2. Description of the Related Art
Conventionally, in an internal combustion engine for use as an automobile engine or a like engine, a spark plug used for igniting an air-fuel mixture generally includes a center electrode inserted into a front end portion of an axial hole formed in an insulator, a metallic shell provided outside the insulator, and a ground electrode whose one end is joined to the metallic shell and whose opposite end faces the center electrode across a spark discharge gap.
Such spark plugs, together with ignition coils, among other components, constitute an ignition apparatus for use in an internal combustion engine. In the ignition apparatus for use in an internal combustion engine, high voltage for ignition is generated at an ignition coil and applied to a spark plug, whereby spark discharge is induced in a spark discharge gap to thereby ignite an air-fuel mixture within a combustion chamber.
A known spark plug is configured such that thread grooves are formed on the outer circumferential surface of the metallic shell for fixedly attaching the spark plug to a cylinder head through screw engagement.
In order to meet recent demands for high efficiency (improvement of fuel economy) and clean emission, some internal combustion engines employ a high air-fuel ratio (lean air-fuel ratio) for an air-fuel mixture. Such an internal combustion engine employing a lean air-fuel ratio involves a problem in that ignition of an air-fuel mixture is impaired when the amount of fuel present in the vicinity of the spark discharge gap is small. In order to cope with this problem, the internal combustion engine employing a lean air-fuel ratio utilizes a turbulent flow (swirl flow or tumble flow) of an air-fuel mixture within a combustion chamber for stabilizing ignition of an air-fuel mixture.
However, when turbulent flow of an air-fuel mixture is generated, the orientation of the ground electrode of a spark plug within a combustion chamber is known to influence ignition of the air-fuel mixture; i.e., when the ground electrode is oriented in a certain direction within the combustion chamber, ignition may be impaired. In connection with ignition of an air-fuel mixture through contact with spark discharge induced in the spark discharge gap of a spark plug, for example, when the ground electrode is located upstream of the spark discharge gap with respect to a turbulent flow of the air-fuel mixture, the ground electrode serves as a block and hinders contact between the air-fuel mixture and spark discharge. As a result, ignition of the air-fuel mixture may be impaired.
In the case of a spark plug which is attached to a cylinder head through screw engagement, the orientation of a ground electrode within a combustion chamber differs among cylinders and is thus uncertain; therefore, difficulty is encountered with such a spark plug in orienting a ground electrode in a specific direction. Also, in the case of a spark plug which is attached to a cylinder head through screw engagement, a slight dimensional error of thread grooves or individual differences cause variation of the orientation of a ground electrode within a combustion chamber at the final stage of attachment. Therefore, such a spark plug encounters difficulty in orienting a ground electrode in a specific direction.
Thus, according to a contemplated structure, a spark plug is loosely fitted into a cylinder head without forming thread grooves for fixation on a metallic shell of the spark plug and without forming internal thread grooves in the cylinder head for engaging the spark plug. Employment of a structure in which a spark plug is loosely fitted into a cylinder head for attachment can avoid variations in the orientation of a ground electrode within a combustion chamber among spark plugs and can facilitate orientation of a ground electrode in a specific direction within a combustion chamber. However, in contrast to a conventional structure in which a spark plug is attached to a cylinder head through screw engagement, in the devised structure, gastightness (seal) must be maintained between a spark plug and a cylinder head, and a spark plug must be held fixedly attached to the cylinder head in a stable condition when combustion pressure is applied to the spark plug from a combustion chamber.
It is therefore an object of the present invention to provide an ignition apparatus for use in an internal combustion engine, which facilitates orientation of a ground electrode within a combustion chamber at the final stage of attachment of a spark plug to a cylinder head, exhibits good gastightness between the spark plug and the cylinder head, and is capable of fixedly attaching the spark plug to the cylinder head in a stable condition by a simple structure.
In order to achieve the above object, the invention provides an ignition apparatus for use in an internal combustion engine, comprising a spark plug, which comprises a center electrode inserted into a front end portion of an axial hole formed in an insulator, a metallic shell provided outside the insulator, and a ground electrode joined to the metallic shell and forming a spark discharge gap between the ground electrode and the center electrode and which is disposed in a plug disposition hole formed in a cylinder head of the internal combustion engine, the ignition apparatus being adapted to induce spark discharge in the spark discharge gap of the spark plug for igniting an air-fuel mixture. The ignition apparatus is characterized in that the metallic shell of the spark plug comprises a flange portion having a plug seat formed on a front end face thereof and adapted to come in contact with the cylinder head directly or indirectly via another member, and a cylindrical front end portion extending axially frontward from the plug seat of the flange portion and having no thread grooves formed on an outer circumferential surface thereof, and in that a plug hole pipe, which is disposed rearward in relation to the plug seat of the metallic shell and is fixedly attached to the cylinder head, causes the plug seat of the metallic shell to be pressed against the cylinder head, whereby the spark plug is fixedly attached to the cylinder head.
A spark plug to be mounted on this ignition apparatus for use in an internal combustion engine has no thread grooves formed on the outer circumferential surface of the front end portion of the metallic shell; i.e., the ignition apparatus does not employ screw engagement for attaching the same to a cylinder head. The front end portion of the metallic shell has no thread grooves formed on the outer circumferential surface thereof, but is formed into a cylindrical shape, whereby the spark plug can be attached to the cylinder head while being loosely fitted into the cylinder head. Thus, when the spark plug is attached to the cylinder head, a position on the metallic shell where the ground electrode is joined can be readily set to a circumferentially specific position about the axis of the plug disposition hole (i.e., the ground electrode can be oriented in a specific direction within a combustion chamber), thereby preventing uncertain orientation of a ground electrode as is the case where a spark plug is screw-engaged with a cylinder head. For example, this spark plug allows orientation of the ground electrode in the following manner: the orientation of the ground electrode within a combustion chamber is set beforehand; and the spark plug is inserted into a cylinder head while the orientation of the ground electrode is maintained, and is then fixed, whereby at the final stage of attachment the ground electrode can be oriented in a specific direction within the combustion chamber.
The direction of a turbulent flow of an air-fuel mixture in a combustion chamber depends on the structure of an inlet pipe and that of a combustion chamber and is thus substantially constant with respect to a cylinder head. Therefore, when the spark plug is to be attached to the cylinder head, by setting beforehand the orientation of the ground electrode within the combustion chamber in view of the direction of a turbulent flow of an air-fuel mixture, the ground electrode can be oriented within the combustion chamber in a specific direction in relation to the direction of a turbulent flow of an air-fuel mixture. In contrast to a conventional practice, the plug disposition hole does not require thread grooves, and therefore the diameter (inside diameter) of the plug disposition hole can be reduced, thereby facilitating disposition of large-area inlet and exhaust valves on the cylinder head and facilitating design for such disposition.
The ignition apparatus for use in an internal combustion engine of the present invention is characterized in that, when a spark plug including a metallic shell whose front end portion has no thread grooves formed on an outer circumferential surface thereof and is formed into a cylindrical shape as described above is to be fixedly attached to a cylinder head, a plug hole pipe, which is disposed rearward in relation to the plug seat of the metallic shell and fixedly attached to the cylinder head, is used for the fixation, and is further characterized in that the plug hole pipe causes the plug seat to be pressed against the cylinder head. Notably, the term xe2x80x9crearwardxe2x80x9d used in relation to a spark plug refers to the side toward an axial end of the spark plug opposite the front end of the spark plug.
Incidentally, an intake-valve-exhaust-valve drive mechanism including cam shafts and adapted to drive intake and exhaust valves is provided on an upper portion of a cylinder head of an internal combustion engine. In order to smoothly operate the intake-valve-exhaust-valve drive mechanism, lubricating oil is circulated within the upper portion of the cylinder head. Since high voltage for ignition generated by an ignition coil is applied to a spark plug for induction of spark discharge, adhesion of lubricating oil raises a problem in that spark discharge cannot be induced normally, because of electrical leakage or a like cause. In order to cope with this problem, the internal combustion engine employs a plug hole pipe whose inside diameter is determined so as to allow insertion of a spark plug thereinto and which is provided on the upper portion of the cylinder head where the intake-valve-exhaust-valve drive mechanism is disposed, thereby providing a spark plug disposition space and isolating the spark plug from lubricating oil. The plug hole pipe is fixedly attached to the cylinder head such that the axis thereof is aligned with that of the spark plug.
In the present invention, the plug hole pipe is used not only for isolating a spark plug from lubricating oil by determining an inside diameter thereof such that the same can enclose the spark plug, but also as a fixation member for fixedly attaching the spark plug to a cylinder head.
By using a plug hole pipe to be attached to a cylinder head as a fixation member for fixedly attaching, to the cylinder head, a spark plug whose metallic shell has no thread grooves formed thereon, the number of components can be reduced as compared with the case where a fixation member for fixing a spark plug is newly added. Since a special fixation member is not employed, manufacturing cost can be reduced, and a spark plug whose metallic shell has no thread grooves formed thereon can be fixedly attached to a cylinder head by use of a simple structure.
In contrast to a conventional structure in which a spark plug is attached to a cylinder head through screw engagement, when a spark plug whose metallic shell has no thread grooves formed thereon is fixedly attached to a cylinder head, gastightness (seal) must be maintained between the spark plug and the cylinder head, and the spark plug must be held fixedly attached to the cylinder head in a stable condition even when combustion pressure is applied to the spark plug from a combustion chamber, or the internal combustion engine itself undergoes strong vibration.
According to the present invention, in order to maintain gastightness between a spark plug and a cylinder head, a plug hole pipe is fixedly attached to the cylinder head such that the plug seat of a metallic shell is pressed against the cylinder head, and the plug hole pipe causes the spark plug to be fixedly attached to the cylinder head. As a result, the plug seat of the flange portion of the metallic shell is uniformly pressed against the cylinder head along the circumferential direction, whereby the plug seat and the cylinder head can maintain stable contact with each other. Further, since the plug hole is fixedly attached to the cylinder head such that the plug seat of the metallic shell is pressed against the cylinder head, even when combustion pressure is applied to the spark plug from a combustion chamber, the spark plug can be held fixedly attached to the cylinder head in a stable condition.
Preferably, in order to fixedly attach the spark plug to the cylinder head in a stable condition by use of the plug hole pipe, the plug hole pipe is attached to the cylinder head such that the plug seat of the metallic shell is pressed against the cylinder head by a pressing force greater than combustion pressure generated by combustion of an air-fuel mixture; specifically, by a pressing force of not less than 10 MPa.
Thus, according to the ignition apparatus for use in an internal combustion engine of the present invention, at the final stage of attachment of the spark plug, the ground electrode can be oriented within the combustion chamber in a specific direction in relation to the direction of a turbulent flow of an air-fuel mixture. Also, the plug hole pipe is used not only for isolating the spark plug from lubricating oil, but also as a fixation member for fixedly attaching the spark plug to the cylinder head, whereby the spark plug, whose metallic shell has no thread grooves formed thereon, can be fixedly attached to the cylinder head in a stable condition and in a manner such that contact between the spark plug and the cylinder head is favorably maintained even when vibration of the internal combustion engine is propagated to the spark plug.
Preferably, when the plug hole pipe is to be fixedly attached to the cylinder head such that the plug seat of the metallic shell is pressed against the cylinder head, the plug hole pipe to be disposed rearward in relation to the plug seat of the metallic shell abuts and presses a portion of the metallic shell to thereby fixedly attach the spark plug to the cylinder head.
By fixedly attaching the plug hole pipe to the cylinder head such that the plug hole pipe abuts and presses a portion of the metallic shell as described above, the plug seat of the metallic shell can be reliably pressed against the cylinder head. Thus, the spark plug, whose metallic shell has no thread grooves formed thereon, can be fixedly attached to the cylinder head in a stable condition and in a manner such that contact between the spark plug and the cylinder head is favorably maintained even when vibration of the internal combustion engine is propagated to the spark plug.
Preferably, when the plug hole pipe is caused to abut a portion of the metallic shell, the plug hole pipe accommodates a portion of the spark plug which extends rearward from the rear end face of the flange portion of the metallic shell, the rear end face being an end face located opposite the plug seat, and abuts and presses the rear end face of the flange portion of the metallic shell to thereby fixedly attach the spark plug to the cylinder head. Thus, the plug seat of the metallic shell can be reliably pressed against the cylinder head by use of the plug hole pipe.
Preferably, in the case of a spark plug whose metallic shell comprises a crimped portion, which is formed by crimping a circumferential edge of a rear end portion thereof toward an outer circumferential portion of an insulator, the plug hole pipe accommodates a portion of the spark plug which extends rearward from the crimped portion of the metallic shell, and abuts and presses the crimped portion to thereby fixedly attach the spark plug to the cylinder head. Thus, the plug seat of the metallic shell can be reliably pressed against the cylinder head by use of the plug hole pipe. Additionally, as compared with the above-described structure in which the plug hole pipe is caused to abut the end face of the flange portion, the outside diameter of the plug hole pipe can be reduced, thereby reducing a space which the plug hole pipe occupies within the cylinder head, and thus increasing a space for disposing other devices, such as an intake-valve-exhaust-valve mechanism.
Preferably, in the above-described ignition apparatus for use in an internal combustion engine, in which the spark plug is fixedly attached to the cylinder head by use of the plug hole pipe, the plug hole pipe is fixedly attached to the cylinder head through press fitting.
In conventional internal combustion engines, press fitting is the most widely used method for fixedly attaching a plug hole pipe to a cylinder head, and the plug hole pipe can be firmly attached to the cylinder head through press fitting. Thus, the spark plug can be pressed against the cylinder head by a pressing force greater than combustion pressure, and the spark plug, whose metallic shell has no thread grooves formed thereon, can be fixedly attached to the cylinder head in a reliable condition.
Once a plug hole pipe is fixedly attached to a cylinder head through press fitting, removal of the attached plug hole pipe is difficult. Therefore, this press-fitting method is not suited for an internal combustion engine which requires periodic replacement of spark plugs.
Thus, preferably, in the above-described ignition apparatus for use in an internal combustion engine, in which the spark plug is fixedly attached to the cylinder head by use of the plug hole pipe, the plug hole pipe comprises a flange portion projecting outward from an outer circumferential surface thereof, and the flange portion is fixedly attached the cylinder head, whereby the plug hole pipe is fixedly attached to the cylinder head.
Utilizing the flange portion for fixedly attaching the plug hole pipe to the cylinder head eliminates the need to press-fit the plug hole pipe into the cylinder head. By fixedly attaching the flange portion to the cylinder by a method that allows removal of the attached flange portion, the fixedly attached plug hole pipe can be removed from the cylinder head.
The flange portion may be fixedly attached to the cylinder head, for example, by use of bolts or the like or by a method in which an engagement portion is provided on the cylinder head and used such that, after a spark plug is held between the plug hole pipe and the cylinder head, the plug hole pipe is turned about the center axis thereof to thereby engage the flange portion with the engagement portion.
Preferably, in the above-described ignition apparatus for use in an internal combustion engine, in which the spark plug is fixedly attached to the cylinder head by use of the plug hole pipe, the plug hole pipe has thread grooves formed on an outer circumferential surface thereof, and the thread grooves are engaged with thread grooves formed on the cylinder head, whereby the plug hole pipe is fixedly attached to the cylinder head.
This fixation method allows removal of the fixedly attached plug hole pipe from the cylinder head and enables fixation of the plug hole pipe onto the cylinder head without use of additional members such as bolts. Also, through control of torque applied for tightening the plug hole pipe into the cylinder head, a force (load) to be applied to the spark plug by the plug hole pipe can be rendered substantially constant, whereby the plug hole pipe can cause the plug seat of the metallic shell to be pressed against the cylinder head stably and without variations among spark plugs.
Thus, according to the ignition apparatus for use in an internal combustion engine of the present preferred aspect of the invention, the spark plug can be replaced and can be fixedly attached to the cylinder head by use of a small number of components, thereby reducing the cost of manufacturing the internal combustion engine.
Preferably, in the above-described ignition apparatus for use in an internal combustion engine, in which the spark plug is fixedly attached to the cylinder head by use of the plug hole pipe, a cylinder head cover is provided on the cylinder head, and the plug hole pipe is held between the cylinder head cover and the cylinder head to thereby be fixedly attached to the cylinder head.
This fixation method utilizes the cylinder head cover to be provided on the cylinder head, for fixing the plug hole pipe within the cylinder head. Specifically, a force associated with fixation of the cylinder head cover onto the cylinder head with the plug hole pipe being held therebetween is used for fixedly attaching the plug hole pipe to the cylinder head and transmitted to the plug hole pipe, whereby the plug seat of the metallic shell can be reliably pressed against the cylinder head by use of the plug hole pipe.
As discussed above, in the case of the spark plug whose metallic shell has no thread grooves formed thereon, when the spark plug is attached to the cylinder head, a position on the metallic shell where the ground electrode is joined can be readily set to a circumferentially specific position about the axis of the plug disposition hole (i.e., the ground electrode can be oriented in a specific direction within a combustion chamber). However, after the spark plug is inserted into the plug disposition hole, the spark plug may turn with a resultant change in the orientation of the ground electrode within the combustion chamber.
Thus, preferably, in the ignition apparatus for use in an internal combustion engine described above, the spark plug is configured such that the metallic shell comprises an anti-rotation portion which is engaged with the cylinder head at one or more positions when the spark plug is turned about the axis of the plug disposition hole in the course of attachment to the cylinder head; the cylinder head comprises an anti-rotation reception portion corresponding to the anti-rotation portion; and when the spark plug is fixedly attached to the cylinder head, the position of the joint between the metallic shell and the ground electrode is aligned with a predetermined circumferential position about the axis of the plug disposition hole.
The anti-rotation portion formed on the metallic shell and the anti-rotation reception portion formed on the cylinder head are fitted to or engaged with each other to thereby disable the spark plug turning about its axis and to thereby specify the orientation of the ground electrode within the combustion chamber when the spark plug is attached to the cylinder head. The anti-rotation portion may assume the form of, for example, a protrusion extending axially on the outer surface of a flange portion or front end portion of the metallic shell or a groove formed axially on the outer surface of the flange portion or front end portion of the metallic shell. Alternatively, the flange portion or front end portion of the metallic shell may include a special-section portion whose section taken along a plane perpendicular to the axial direction of the spark plug assumes a shape other than circle (polygon (such as hexagon), oval, etc.) and which serves as an anti-rotation portion.
In this case, the anti-rotation reception portion capable of being fitted to or engaged with the above-described anti-rotation portion is provided on the cylinder head such that the ground electrode is oriented in a direction of good ignition within the combustion chamber. Thus, merely by fitting (engaging) the anti-rotation portion of the metallic shell to (with) the anti-rotation reception portion formed on the cylinder head, the ground electrode can be reliably and readily oriented in a predetermined direction of good ignition within the combustion chamber.
Thus, according to the ignition apparatus for use in an internal combustion engine of the present preferred aspect of the invention, after the ground electrode is oriented in a certain direction within the combustion chamber in the course of attachment of the spark plug to the cylinder head, a change in the orientation of the ground electrode can be prevented, and the ground electrode can be readily and reliably oriented in a specific direction within the combustion chamber, whereby good ignition can be attained.
Incidentally, a plug hole pipe is originally intended to isolate a spark plug from lubricating oil. As a result of adding to the plug hole pipe a function for fixedly attaching a spark plug to a cylinder head, the structure of the plug hole pipe becomes complicated to a certain degree as compared with a conventional plug hole pipe. Therefore, a dimensional error or a like factor may cause formation of clearance between the plug hole pipe and the cylinder head, potentially resulting in penetration of lubricating oil.
Thus, preferably an oil seal member is provided between the plug hole pipe and the cylinder head. The oil seal member prevents adhesion of lubricating oil to the spark plug, thereby preventing occurrence of an anomaly, such as the spark plug""s failing to induce normal spark discharge, which would otherwise result from adhering lubricating oil. The oil seal member is formed of, for example, an elastically deformable material (rubber, etc.). The sectional shape of the oil seal member is deformed according to clearance formed between the plug hole pipe and the cylinder head to thereby fill the clearance, thereby favorably preventing penetration of lubricating oil.
Preferably, in the ignition apparatus for use in an internal combustion engine described above, the plug seat of the metallic shell of the spark plug is in contact with the cylinder head via a gasket.
By employing a gasket, when the spark plug is fixedly attached to the cylinder head such that the plug hole causes the plug seat of the metallic shell to be pressed against the cylinder head, gastightness between the spark plug and the cylinder head can be maintained more reliably. Also, since a heat release path via the gasket is established, the spark plug""s performance (particularly heat resistance) can be retained.